Sulfamethoxazole (SMX), sulfadiazine, dapsone, pentamidine, and procainamide are clinically important drugs, the use of which is limited by the occurrence of hypersensitivity reactions in some patients. These reactions are thought to be related to the presence of a toxic hydroxylamine metabolite and its spontaneous byproduct, the nitroso metabolite. Unlike the parent compounds, these metabolites are both cytotoxic and immunogenic. Hypersensitivity reactions to SMX and related drugs occur in a high percentage (25-60 percent) of patients with AIDS. The reason for this high incidence is unclear, but data from in vitro cytotoxicity assays in peripheral blood mononuclear cells (PBMC's) suggest that both HIV-positive and HIV-negative patients with such hypersensitivity have a defect in hydroxylamine or nitroso detoxification. We hypothesize that detoxification of hydroxylamine and nitroso metabolites by reduction is an important determinant of hypersensitivity to sulfonamide and related drugs. We further hypothesize that NADH cytochrome b5 reductase, which is involved in hydroxylamine reduction in some species, is important for hydroxylamine detoxification in humans. In addition, we have novel data to suggest that flavins (FAD and FMN); in the presence of glutathione, are capable of non- enzymatic reduction of hydroxylamines. We therefore propose that defects in either NADH- or flavin-dependent reduction of hydroxylamines and nitroso metabolites are associated with the outcome of sulfonamide hypersensitivity. To address these hypotheses, we will examine the role of NADH cytochrome b5 reductase (b5R) in hydroxylamine and nitroso reduction in both liver and PBMC's, using expressed human recombinant b5R and antibodies to human b5R. We will evaluate the role of flavins in hydroxylamine and nitroso reduction using in vitro stoichiometric assays in a cell-free system, and correlation of flavin and glutathione content with activity in hepatic microsomes and PBMC's. Finally, we will correlate hydroxylamine and nitroso reduction, b5R expression, and flavin content with the outcome of SMX hypersensitivity in patients with HIV infection. The ultimate goal of these studies is to better understand the pathogenesis of toxicity to sulfonamides and a related group of clinically important drugs which generate hydroxylamine and nitroso metabolites. The results of these experiments, which will identify the pathways involved in the detoxification of these metabolites, will suggest better strategies for the prevention of hypersensitivity and other hydroxylamine-related toxicity.